Until fairly recently, the preferred, indeed the only means by which to display information in the electronic medium was to use a video monitor comprising a cathode ray tube ("CRT"). CRT technology has been well known for over 50 years, and has gained widespread commercial acceptance in applications ranging from desktop computer modules to home televisions and industrial applications. CRTs are essentially large vacuum tubes having one substantially planar surface upon which information is displayed. Coated on the inside surface of the CRT is a layer of phosphorous which respond by emitting light when struck by electrons emitted from the electron gun of the CRT. The electron gun is disposed in an elongated portion which extends away from the inside surface of the display surface of the CRT.
While CRTs are widely used in numerous applications, there are several inherent limitations to the application of CRT technology. For example, CRTs are relatively large and consume a great deal of energy. Moreover, as there are fabricated of glass, the larger they get the heavier they get. Given the need for the electron gun to be spacedly disposed from the phosphorous surface of the display surface, CRTs have a substantial depth dimension and width dimensions thereof. Accordingly, CRTs are absolutely of no value for a small and portable applications, such as Walkmen, laptop computers, and other increasingly portable electronic applications which require the use of displays.
To answer the needs of the marketplace for smaller, more portable display devices, manufacturers have created numerous types of flat panel display devices. Examples of flat panel display devices include active matrix liquid crystal displays, plasma displays, and electroluminescent displays. Each of these types of displays has use for a particular market application, though each are accompanied by various limitations which make them less than ideal for certain applications. Principal limitations inherent in devices such as AMLCDs relate to the fact that they are fabricated predominantly of inorganic semiconductor materials by semiconductor fabrication processes. These materials and processes are extremely expensive, and due to the complexity of the manufacturing process, cannot be reliably manufactured in high yields. Accordingly, the costs of these devices are very high with no promise of immediate cost reduction.
One preferred type of device which is currently receiving substantial research effort is the organic electroluminescent devices. Organic electroluminescent devices ("OED") are generally composed of a plurality of layers of organic molecules sandwiched between transparent, conductive and/or metallic conductive electrodes. There are typically three organic layers which include an electron transporting layer, an emissive layer, and a hole transporting layer. Charge carriers, i.e., electron and holes, inject from either the electron or hole transporting layers, and combine in the emissive layer. Electrons are negatively charged atomic particles and holes are the positively charged counterparts.
There are several variations in OED structures, depending upon where the emissive layer is positioned. Tsutsui, et al proposed three OED cell structures: SH-A, SH-B, and DH. (T. Tsutsui, et al, Photochem. Processes Organ. Mol. Syst., Proc. Meml. Conf. Late Professor Shigeo Tazuke, 437-50 (1991)). SH-A cells are successively composed of a plurality of layers including Mg-Ag as a cathode electrode, an electron transporting layer (ETL), a hole transporting layer (HTL), and indium tin oxide or ITO as the anode electrode. The region of the ETL close to the HTL is doped with an efficient, thermally stable fluorescent dye to act as the emitter region or layer. An SH-B type cell likewise comprises a Mg--Ag as a cathode electrode, an ETL, an HTL, and ITO as the anode electrode. However, unlike the SH-A, the region of the HTL close to the ETL is doped with an efficient, thermally stable fluorescent dye to act as the emitter region or layer. Finally, the DH type of display again comprises Mg--Ag as a cathode electrode, an ETL, a HTL, and ITO as the anode electrode. The emitter region or layer in a DH cell is a discrete layer of an emitter material operatively disposed between the ETL and the HTL.
U.S. Pat. No. 4,539,507 to VanSlyke, et al, is among the first to disclose an SH-A type display with a hole injecting and luminescent zone. Subsequent patents to VanSlyke and others have disclosed devices and materials which are adapted to provide OEDs which lumenese in the blue to blue-green portions of the spectrum. In this regard, reference is made to, among others, U.S. Pat. Nos. 5,150,006, 5,141,671, 5,151,629, and 5,153,073.
Commonly assigned, co-pending U.S. patent application Ser. No. 08/660,014, filed Jun. 6, 1996, and entitled "ORGANIC ELECTROLUMINESCENCE DEVICE WITH EMISSION FROM HOLE TRANSPORTING LAYER", discloses an efficient SH-B type of OED, where, with proper selection of the ETL and HTL materials, efficient light emission is obtained from the HTL. To realize the disclosed structure, an efficient emissive hole transporting material, which can fluoresce well in the blue to green region of the spectrum, i.e., 450-550 nanometers (nm), is required.
Accordingly, there exists a need for emissive hole transporting materials for use as the hole transporting layer in OEDs. The material should be relatively inexpensive and easy to fabricate as well as being conducive to manufacturing in the current OED manufacturing process. The device should have good thermal stability, and be capable of operating at voltages which are within the range of those generally accepted for OEDs, and fluoresce will in the blue to blue-green region of the spectrum.